Polymeric membranes, suitable for use in ultrafiltration and reverse-osmosis processes, are prepared typically by casting, or otherwise forming, the membrane in sheet, tubular, hollow-fiber, spiral or other form from a casting solution containing the desired polymer. It is often desirable, depending upon the particular ultrafiltration or reverse-osmosis process, to prepare polymeric membranes having either positive or negative functional groups incorporated into the polymeric membrane, or to form polymeric membranes having a blend of different polymers, in order to provide different functional characteristics to the resulting polymeric membrane.
A number of difficulties associated with the preparation of blended polymeric membranes is that, in some cases, the direct blending of the polymers or the grafting of monomers onto a matrix-supporting polymer is inhibited, due to the limited choices of desirable solvents for use in the casting solution, or, where the casting solution is formed, the solution is noncastable or nonusable, due to gelation of the casting solution, physical separation of one or more of the ingredients of the casting solution, or other incompatibilities. Another difficulty associated with the preparation of blended polymeric membranes is related to the water solubility of some of the polymers employed in the casting solution. Thus, where a particular, water-soluble homopolymer or copolymer is blended in with another polymer in the casting solution to prepare a blended-polymer membrane, the water-soluble polymer may be leached out of the blended membrane, for use in the ultrafiltration or reverse-osmosis process. A further problem associated with the preparation of blended polymeric membranes occurs after the casting solution is prepared and the membrane formed from the casting solution. It is sometimes found that, in the membrane so formed, on inspection and after evaporation or diffusion of the casting solvent, the polymers making up the membrane are totally incompatible. Thus, for example, while in solution, two or more polymers may be compatible, but, in solid, dry or semidry form, the polymers are incompatible, and, on drying, may provide a visual appearance of incompatibility, such as a white, chalky film or cracks or crazing on the surface, or the membrane may have nonvisual effects and otherwise fail to form a proper membrane suitable for use in an ultrafiltration or reverse-osmosis process. Incompatibility in the dry, blended membrane often indicates that, during coacervation of the casting solution as the membrane is formed, the polymers separate in separate microregions, mostly between the polymeric forms, instead of forming a blended, compatible, water-insoluble, interpenetrating polymer network, which prevents substantial leaching of the water-soluble polymer, and provides for the formation of a compatible, blended polymer.
It is desirable to provide for blended polymeric membranes of desired polymeric characteristics and functional groups, which may be blended into a matrix or supporting polymer in an easy, rapid, effective and economical manner, without the difficulties associated with the preparation of past, blended, polymeric membranes.